WHY TWO-BLADED ROTORS MAKE SENSE

A properly designed two bladed rotor can substantially reduce these large gyroscopic forces. In most cases, it would be impractical to rigidly mount a two bladed rotor on a free yaw turbine. Because the mass distribution is asymmetrical lying in a straight line – the gyroscopic force varies depending on the position of the rotor during its revolution. So, instead of the relatively constant force of a three blade rotor, the amplitude of the force will vary at a frequency of twice the RPM of the rotor. This can cause severe vibration of the turbine and tower. The two blade vibration problem can be solved by mounting the blades on a teeter hinge. A teeter hinge is simply a pin at the center of the blade where it attaches to the hub. The blade assembly looks and works like a see-saw. An example of a teeter hinge can be seen in the image of the Aerostar™ 6 Meter Wind turbine (www.aerostarwind.com).

The advantage of a teeter hinge is that it decouples the gyroscopic force from the turbine. During the part of the blade revolution where the force is present, the blade rotates on the teeter hinge. By teetering, gyroscopic forces are isolated and not transmitted to the turbine and tower. The result is that, unlike three bladed rotors, two blade teetering rotors do not transmit appreciable gyroscopic forces to the turbine and tower. This allows designers to reduce the weight and cost of major structural components.

https://www.youtube.com/watch?v=XVj_YyvgMbE

https://en.wikipedia.org/wiki/Transfer_molding